Cotton fiber is the single most important textile worldwide. About 80 million acres of cotton are harvested annually across the globe. Cotton is the fifth largest crop in the U.S. in terms of acreage production, with an average of 10.3 million acres planted in the years 2006 to 2008. About 90% of cotton grown worldwide is Gossypium hirsutum L., whereas Gossypium barbadense accounts for about 8%.
However, like other natural cellulose containing fibers, cotton fibers do not possess the chemical versatility of synthetic fibers, due to the relative inert nature of the β-1-4 linked glucose monomers in cellulose. This relative inert nature is e.g. apparent during the dyeing process of cotton fibers and fabrics.
Generally two types of dyes are used to color cotton: direct dyes and fiber-reactive dyes, which are both anionic molecules. Cotton itself develops an anionic charge in water, so that without special treatment, the uptake of dye by the fiber or fabric is quite elaborate. Direct dyes create a relatively weak hydrogen bond with the cellulose polymer forming a semi-permanent attachment. Direct dyes are easier to use and less expensive than fiber-reactive dyes, but do not withstand well washing. Fiber-reactive dyes are molecules that combine chromophores with a reactive group that forms strong covalent bonds with the fiber via reaction with hydroxyl groups. The covalent bonds provide a good resistance of the dyed fiber against laundering. Colorfastness can be improved using cationic fixatives.
During the dyeing process using reactive dyes, large amounts of electrolytes are needed to shield the anionic dyes from the anionic fiber charges. Unreacted dyes (up to 40%) need to be removed by a washing step, generating large volumes of wastewater, also containing the above mentioned electrolytes.
Providing the cellulose fiber with a positive electric charge, e.g. by incorporation of positively charged chemical compounds such as positively charged polysaccharides, could therefore improve the dyeability of natural cellulose fibers, as well as improve any chemical reaction of the modified cellulose fiber with negatively charged chemical compounds. It would also make the use of acidic dyes possible.
Several publications have described the incorporation into or coating of chitosan oligomers into cellulose fibers to make chitosan/cellulose blends, yarns or fabrics. Chitosan is a positively charged polymer of glucosamine, which can be obtained by deacetylation of chitin, e.g. by alkalic treatments. Chitin itself is a polymer of e1-4 linked N-acetylglucosamine (GlcNAc). Based on the physiological function of chitosan in inhibiting e.g. dermatophytes, many functional clothes, fabrics and fibers employ cellulose-chitosan blend fibers, cellulose fiber-chitosan conjugates and fabrics coated with chitosan-containing resins.
US patent application US2003/0134120 describes the coating of natural fibers with chitosan.
Liu et al. (Carbohydrate Polymers 44(2003) 233-238) describe a method for coating cotton fibers with chitosan, by oxidation of the cotton thread with potassium periodate at 60° C. in water and subsequent treatment with a solution of chitosan in aqueous acetic acid. With the chitosan coating, the cotton fiber surface became physiologically and biologically active. Since the chemical reactivity of the amino group is greater than the hydroxyl group of cellulose monomers, the fiber has more potential for further chemical modification. Moreover, the smooth surface of the cotton fiber became coarse, suggesting a greater potential for drug absorption and controlled release thereof.
WO2006/136351 provides methods and means for the modification of the reactivity of plant cell walls, particularly as they can be found in natural fibers of fiber producing plants by inclusion of positively charged oligosaccharides or polysaccharides into the cell wall. This can be conveniently achieved by expressing a chimeric gene encoding an N-acetylglucosamine transferase, particularly an N-acetylglucosamine transferase capable of being targeted to the membranes of the Golgi apparatus in cells of a plant. One of the applications is increased dyeability.
WO2011/089021 provides methods and means for the modification of the reactivity of plant secondary cell walls, particularly in cotton cell walls found in cotton fibers. This can be conveniently achieved by expressing a chimeric gene encoding a Saprolegnia monoica chitin synthase in cotton plants.
WO2012/048807 provides alternative methods and means to produce positively charged oligosaccharides in the plant cell wall by introducing into said plant cell a Nodulation C (NOD C) protein fused to a heterologous Golgi signal anchor sequence.
The polysaccharide chitin is built from N-acetylglucosamine residues. It is synthesized from UDP-N-acetylglucosamine which is the end-product of the hexosamine biosynthesis pathway also active in plants (Mayer et al. 1968, Plant Physiol. 43, 1097-1107). The first and rate limiting step of this pathway is the conversion of glutamine to glucosamine-6-phosphate which is catalyzed by the enzyme glutamine:fructose-6-phosphate-amidotransferase (GFAT).
WO 2007/039314 describes transgenic plants having the activity of a hyaluronan synthase and additionally an increased glutamine:fructose-6-phosphate amidotransferase (GFAT) activity. These plants synthesize an increased amount of hyaluronan compared to plants having only the activity of a hyaluronan synthase. Like chitin, hyaluronan is synthesized from UDP-N-acetylglucosamine.
WO 2011/089021 discloses transgenic cotton plants comprising a chimeric chitin synthase gene and a chimeric glutamine:fructose-6-phosphate-amidotransferase gene under the control of a cotton selective promotor. Fibers from these transgenic cotton plants have an increased amount of N-acetylglucosamine polymers which are evenly distributed throughout the cell wall.
Yet there remains a need for improved methods and means to produce cotton fibers which comprise an increased level of positively charged polysaccharides such as oligo-N-acetylglucosamines or oligo-glucosamines. These and other problems are solved as described hereinafter in the summary, detailed embodiments, examples, drawings and claims.